1. Field of Invention
The present invention relates generally to improved brush seal designs for use in axially flow elastic fluid turbines.
2. Brief Description of the Prior Art
The use of axial flow elastic fluid turbines, such as axial flow steam turbines, plays a very important role in the production of electric power in our society. In order produce electrical power from an electrical power generator installed at a power plant, it is necessary to rotate the rotor shaft thereof in a magnetic field produced by the stator field windings of the power generator. Typically, the torque required to rotate the rotor shaft at a sufficient angular velocity is provided by a steam turbine whose output shaft is mechanically coupled to the rotor shaft of the generator. Often, in a typical power plant, there will be a number cf steam turbines each driving one or more electrical power generators.
In general, each steam turbine comprises a shaft rotatably supported by bearings which are encased in a housing or casting typically referred to as the turbine shell. In order to rotate the rotor shaft using the momentum of super-heated vapor (i.e., "steam"), a series of turbine stages are sequentially arranged along the axis of the shaft. A boiler, typically located external to the. turbine casting, is provided for the purpose of generating steam. External to the turbine casting are steam pipes which are used to conduct the steam from the boiler to particular sections of the turbine, that are typically classified by operating pressure. Along each section of the turbine, there are typically a number of turbine stages.
At each turbine stage, a turbine rotor is fixedly mounted to the rotor shaft. Each turbine rotor has a plurality of blades which radially extend a predetermined distance from the shaft, towards a circumferentially extending shroud band (i.e., cover) that is secured to the tenon portions of the blades. In general, each turbine blade is oriented at an acute angle with respect to the axis of rotation of its rotor. In order that each turbine rotor is permitted to freely rotate with the turbine shaft, the turbine casting has circumferential recesses to accommodate the rotor structures along the shaft. A stationary diaphragm is installed behind each rotor in a circumferential joint formed in the turbine casting. Each turbine has a ring of steam nozzles circumferentially extending about the inner structure of the diaphragm. These nozzles are located at the same radial position as the blades in its associated rotor. The function of each nozzle is to receive steam from the passageways in the turbine casting and to physically direct the steam against the rotating blades of its associated rotor. To establish a "tip seal" with the shroud band of each turbine rotor, a ring of spillstrips segments are supported from the diaphragm at each stage.
As the steam travels through the turbine, a portion of its linear momentum is transformed into the angular momentum of the rotor blades at each turbine stage, thereby imparting torque to the turbine shaft (i.e., rotor shaft). At each subsequent stage, the pressure of the steam path is typically reduced. Thus at these downstream stages it is often necessary to increase the length of the rotor blades and the size of the associated diaphragms in order to extract kinetic energy from axially flowing steam of reduced pressure.
A major problem in turbine design relates to the quality of steam seals between the various stationary and rotating components along the steam flow path in the turbine. In general, there are several locations within the turbine where such seals must be established to ensure high turbine efficiency.
The first location where steam seals are required is between the outer portion of each rotor and its associated diaphragm have been effected using a segmented spillstrip ring of the type disclosed in U.S. Pat. No. 5,547,340, incorporated herein by reference. While this Patent discloses an spillstrip design that offers improvements over prior art designs, it is not without its shortcomings and drawbacks. In particular, during start-up operations when the rotor exhibits low frequency modes of operation about its axis, the tips of the rigid fin-like structure (e.g., fin seals) projecting along the spillstrip segments tend to rub against and/or cut into the shroud-band of the associated rotor, causing damage thereto during the start-up process. The only safeguards offered against such rubbing action has been to design the spillstrips so that there exists sufficient clearance between the tip portions of the fins on the spillstrips and the shroud band of the rotor. This approach, however, results in a degrading of the tip seal, allowing steam to pass through the clearance area, and not through and over the blades of the rotor, thereby reducing the performance of the turbine.
The second location where steam seals are required is between the rotor and the turbine shaft. Creating seals over such regions has been addressed along time ago by installing a segmented packing ring structure between the rotor and the turbine shaft at each turbine stage. Each packing ring structure consists of a first ring structure with multiple rows of fins (i.e., seal teeth) and a second ring structure with multiple rows of surface projections. The first ring structure is mounted from the associated diaphragm, whereas the second ring structure is mounted to the turbine shaft. Together, the rows of fins and projection structures create a labyrinth-type seal which presents a high impedence flow path to pressurized steam. However, during start up operation, low frequency modes of operation about the turbine axis tends to cause the tip portions of each row of fins to move radially outwards and inwards. To avoid tip rubbing and damage to such packing ring structures, it is necessary to design the fins and surface projections with sufficient clearance to avoid tip rubbing during start-up operation. This, however, necessarily degrades the quality of the labyrinth seal.
In U.S. Pat. Nos. 4,436,311 and 5,395,124 to Brandon, the problem of fin tip rubbing in packing ring design has been addressed by providing a retractable segmented packing ring structure between each rotor and turbine shaft. The manner in which the quality of the labyrinth seal is improved with this design is described as follows. During startup operation, when low frequency rotor vibration is predominant, the diaphragm-mounted packing ring segments are spring-biased in a radial direction away from the turbine shaft, reducing the risk of fin-tip portion rubbing and packing ring damage. As the rotor increases its angular speed, low frequency vibration is naturally reduced, and the packing ring segments are forced to move closer to the turbine shaft by steam pressure, improving the quality of the labyrinth seal among the rows of fins and surface projections in the packing ring structure, thereby improving the efficiency of the turbine.
An alternative solution to the problem of fin tip rubbing in packing ring designs is disclosed in UK Patent Application Publication No. GB 2 301 635 A. In this UK Patent Publication, a brush-type element is installed between a pair of firs extending from the packing ring segments mounted on the diaphragm. The function of the brush seal is to improve the quality of the labyrinth seal during all phases of operation. A major shortcoming with this design, however, is that during startup operations it does not provide a way of protecting the tips portions of the fin seals without designing a high degree of clearance into the design. Consequently, by virtue of such increased clearance requirements, the quality of the labyrinth seal provided by this prior art packing seal design is necessarily compromised. In addition, the brush seal design disclosed in this prior art publication is very expensive to manufacture, decreasing the price-to-performance ratio of this packing ring structure.
Thus, there is great need in the art for an improved brush seal design for providing high quality tip seals and labyrinth seals in axial flow elastic fluid turbines, while avoiding the shortcomings and drawbacks of prior art technologies.